1. Field of the Invention
The present invention relates to production of delay vibrato in a computor organ of the type disclosed in U.S. Pat. No. 3,809,786.
2. Description of the Prior Art
Vibrato is a frequency modulation of a musical tone, typically at a rate of from about 5Hz to 8Hz. When playing some instruments, notably the violin, the musician will not always introduce vibrato. Thus when playing fast, no vibrato is used. However, when a long sustained note is played, the violinist first sounds the note without vibrato, then gradually adds the vibrato by vibrating his finger on the string. This gradual introduction is called "delayed vibrato" or "string vibrato", and it is a principal object of the present invention to implement such delayed vibrato in an electronic musical instrument.
In the inventor's COMPUTOR ORGAN disclosed in the above mentioned U.S. Pat. No. 3,809,786 musical tones are generated by computing in real time the amplitudes at successive sample points of a complex musical waveshape, and converting these amplitudes to musical sounds. Such computations are carried out each time that a keyboard key is depressed. The fundamental frequency of the generated tone is proportional to a frequency number R that is selected by the depressed key. The manner in which the computations are carried out to generate the musical tones is completely set forth in the cited U.S. Pat. No. 3,809,786 which is incorporated herein by reference. The instrument disclosed in that patent herein is called the "patented computor organ" or simply the "computor organ."
Another object of the present invention is to implement delayed vibrato in such a computor organ. Thus, using the circuitry disclosed herein, whenever a key is depressed the selected note first will be generated without vibrato. If the key is kept depressed for longer than a selectable delay period T.sub.D, vibrato will start to be introduced. The depth of vibrato will increase gradually, until at a time T.sub.V the full vibrato depth will be reached. Thereafter vibrato will continue until note production ceases after the key has been released. This effect is illustrated in FIG. 1.
Typically the vibrato modulation waveshape is a sinusoid, but this is not necessary, and another object of the present invention is to implement vibrato of selectable modulation waveshape. A further object is to permit control of the depth of vibrato, i.e., of the frequency excursion of the vibrato modulation away from the nominal fundamental frequency of the generated tone.